The present invention relates to a method of determining a signal to noise ratio, a method of operating an ion detector, a method of mass spectrometry, a control system for a mass spectrometer and a mass spectrometer.
Mass spectrometers incorporating an ion detector or electron multiplier are well known. It is desirable to arrange the gain of the ion detector such that signals arising from ion arrivals can be clearly differentiated from electronic noise. This is particularly important for orthogonal acceleration Time of Flight mass spectrometers in which many thousands of individual time of flight spectra may be summed to produce a final spectrum. In many cases the arrival rate of ions in each of these spectra may be relatively low and without efficient exclusion of electronic noise, signals arriving from individual ion arrivals at the ion detector may be far less intense than the accumulated or summed electronic noise.
It is advantageous that the signal to noise ratio is maintained at an optimum value such that signals from individual ion arrivals are sufficiently intense so as to be differentiated from electronic noise but not so intense that the dynamic range of the ion detection system is compromised.
It is known to use Time to Digital Converters (“TDC”) and signal averagers or Analogue to Digital Converters (“ADC”) to record the electronic signal produced by an ion detector in response to an ion arrival.
A Time to Digital Converter records an ion arrival event based upon an amplitude threshold. The amplitude threshold may comprise a fixed amplitude threshold or a constant fraction discriminator. The amplitude threshold is set to exclude a high percentage of the electronic noise. The gain of the ion detector is then adjusted to ensure that a significant portion of the signals arising from ion arrivals have a height which exceeds this threshold. This ensures that the majority of ion arrivals result in a recorded event.
A signal averager digitises the signal arising from an ion arrival using an Analogue to Digital Converter. Again it is important to exclude a high percentage of the digitised electronic noise from each time of flight spectrum before summing multiple spectra. Conventionally, an amplitude threshold is used to exclude electronic noise. The threshold may be set to exclude recorded signal below a preset number of least significant bits (“LSBs”) corresponding to the height of the digitised signal.
The gain of the ion detector may then be adjusted to ensure that a significant portion of the digitised signals arising from ion arrivals have a height which exceeds this threshold. This ensures that the majority of ion arrivals result in a recorded event.
The noise may be excluded from the final data by applying a fixed predetermined height threshold or a dynamic or adaptive height amplitude threshold such as is disclosed in U.S. Pat. No. 8,598,513 (Micromass), the contents of which are incorporated herein by reference.
Single ions striking an ion detector give rise to a range of different signal intensities due to the ion detector having an inherent pulse height distribution. This variation in intensity is due to the statistical nature of secondary electron emission associated with an electron multiplier. The average height is related to the mass to charge ratio value and the charge state of the ion as electron yield is related to the velocity and the energy of the ions striking the ion detector. The ion detector gain is usually set for a specific analyte species such that a large proportion (>85%) of all the signals from single ion strikes can be differentiated from electronic noise.
The advantages of peak detecting ADCs are well known and reference is made to US 2009/0321628 (Micromass). In particular, differentiation between signal and noise can be made on the basis of the ion area, for example, in the manner as described in US 2012/0126110 (Micromass), the contents of which are incorporated herein by reference. This method provides superior differentiation between signal and noise thereby allowing lower detector gain and hence a lower digitised signal height can be employed which improves detector lifetime and, importantly, increases the usable dynamic range of the ADC.
It is desired to provide an improved method of operating an ion detector, an improved mass spectrometer and an improved method of mass spectrometry.